This section provides background information related to the present disclosure which is not necessarily prior art.
Aluminum mini-micro-port tubing (MMP) has been used in brazed heat exchangers for automotive condensers and evaporators for the last three decades. The HVAC industry on the other hand predominantly used copper round tubing for heat exchangers. As copper prices saw significant increase in the early 2000s, some HVAC original equipment manufacturers (OEMs) started considering the use of aluminum tubing. Further, field failures of copper heat exchangers from filiform corrosion and significant cost savings propelled the first phase of conversion from copper to aluminum. Unfortunately, HVAC performance requirements were not carefully considered and aluminum MMP that was being successfully used in automotive industry showed a high rate of early field failures in the HVAC market. Several reasons for the early failures include a poor design of the heat exchanger connections and cabinet, the quality of the zinc coating on the MMP tubes, and the aluminum alloy used to manufacture the MMP tube.
Modifications in cabinet design by the OEMs, and improvements in the zinc coating process by MMP manufacturers led to significant improvement in field performance. Further marked improvement came with the research and development of long life alloys from which MMP tubes were manufactured. Specifically, the long life alloy disclosed in U.S. Pat. No. 8,025,748 (widely known as a 31104 alloy) emerged as the dominant long life alloy for the MMP HVAC market.
Under corrosive environments, a 31104 alloy provides a longer life than typical alloys used in this application, such as 1100, 1235, 3102 and 3003 alloys. The corrosion resistance, though, comes at a significant cost premium. A 31104 alloy carries an alloy premium, gives a lower extrusion throughput due to higher flow stress, and lowers extrusion die life. As aluminum penetrates the HVAC heat exchanger market, customers are demanding cost effective solutions. Therefore, there exists a need for a long life corrosion resistant alloy with lower cost and wider availability. This disclosure aims to address the above unfulfilled market demands of lower cost and similar corrosion resistance.
Extruded MMP tube cost is primarily driven by alloy premium, geographical availability and manufacturing cost including extrudability and die life. Extrudability (i.e., ease of extrusion and throughput) and die life primarily result from billet properties and press capability. Billet properties, in turn, are dependent and result from composition and homogenization of the alloy. Another aim of this disclosure, therefore, is to focus on both composition and homogenization in such a way so as to reduce cost by increasing the ease with which the alloy could be processed, and provide similar corrosion resistance to currently available long life alloys.